26 Prof. Volpicelli on Frictional Electricity. 



dia are assumed to be masses superimposed upon the crust, the 

 under surface of which is supposed to be uniform. This seg- 

 ment of the crust is not only treated as a nearly flat plate band, 

 but great inequalities of thickness are assumed ; and the problem 

 is thus invested with circumstances from which it readily follows 

 that a very thick crust becomes necessary in order to compensate 

 such remarkably weakening conditions. As we know that the 

 first of these conditions is decidedly incorrect, and that the other 

 is at least wholly unsupported by facts, we cannot be expected 

 to admit the conclusion to which they would lead. 

 Dublin, June 8, 1859. Henry Hennessy. 



V. On Frictional Electricity. By Prof. Volpicelli*. 



ABOUT a year ago Prof. Volpicelli drew attention to certain 

 striking anomalies observed by him on attempting to 

 develope electricity by the friction of certain resinous substances. 

 After repeating his experiments with a great many different 

 bodies, and thus familiarizing himself more perfectly with the 

 anomalies in question, he is at length enabled to state more pre- 

 cisely the conditions of their existence. 



If a stick of ordinary sealing-wax be held by one extremity 

 and pulled briskly between the finger and thumb of the other 

 hand, no matter whether the latter be naked or covered with a 

 woollen or other glove, negative electricity will in general be de- 

 veloped, as may be seen on presenting the stick thus rubbed to 

 the knob of a dry-pile electroscope. On varying the energy of 

 the stroke, however, it will be found that the indications of the 

 electroscope are by no means constant. For example, negative 

 electricity having been developed in the manner described, let 

 the stick be exposed a second time to friction and in the same 

 manner, the energy of the stroke alone being diminished ; the 

 negative character of the developed electricity will be also dimi- 

 nished ; in fact, on repeating the stroke last described, and after 

 each stroke testing the nature of the electricity, the stick may 

 be reduced to the neutral state, and subsequently converted into 

 a positively electrified body. 



A little practice is alone necessary in order to obtain these 

 effects with certainty, and to have the stick of sealing-wax per- 

 fectly under control, so that by a single stroke of greater or 

 less energy, a positive, negative, or neutral electric condition 

 may be imparted to the sealing-wax, whatever may have been 

 the previous state of the same. Continuing the friction with 



* Abstract of a communication made to the Academia de' Nuovi Liticei 

 of Rome, February 6, 1859. 



